1. Field of the Invention
The present invention relates to a method of regenerating wavelength division multiplex optical signals.
2. Description of the Prior Art
Total mastery of the wavelength division multiplex (WDM) fiber optic transmission technology is an important factor in meeting the growing requirement for very high bit rates for transmitting data.
Wavelength division multiplexing (WDM) combines onto one fiber a plurality of modulated channels with different carrier frequencies. The global bit rate of a transmission line is therefore equal to the sum of the bit rates of the various channels.
However, WDM optical signals are subject to limitations which reduce the propagation distance and affect the quality of the optical signals transmitted.
Such limitations include, in particular:
line losses,
jitter (uncertainty in the timing of the arrival of bits),
accumulation of spontaneous amplification emission noise, in particular in in-line optical amplifiers such as erbium-doped fiber amplifiers, and
non-linear effects, such as phase automodulation or cross phase modulation.
Line losses are generally compensated by in-line optical amplification of the WDM signals, for example using an erbium-doped fiber amplifier (EDFA).
However, in-line amplification in itself cannot alleviate the other limiting factors mentioned above. These remain or are even simulated by in-line optical amplification, for example the accumulation of noise.
This is why it is necessary to regenerate the optical signals at regular intervals. Regeneration entails applying various processes to the optical signals, such as:
retiming the optical signals to prevent jitter,
reshaping the optical signals to their original envelope, which also entails eliminating noise, and
reamplifying the optical signals to their original amplitude levels.
A regenerator that performs the above three actions on an optical signal is usually referred to as a 3R regenerator (Retiming, Reshaping, Reamplifying). One which performs only the first two actions is referred to as a 2R regenerator (Retiming, Reshaping).
For example, the document Nakazawa et al. (1991), xe2x80x9cExperimental demonstration of soliton data transmission over unlimited distances with soliton control in time and frequency domainxe2x80x9d, Electronics Letters, V. 29, No 9, pp. 729-730, Apr. 29, 1993 discloses a regenerator acting on optical signals in the form of solitons through an LiNO3 optical modulator.
The prior art modulator, which is shown diagrammatically in FIG. 1, synchronously modulates incoming optical signals.
To this end, the LiNO3 modulator is controlled by an electronic control signal generated from the in-line optical signal in a clock circuit. The clock recovery means include an optical coupler C3 for extracting part of the optical signal propagating between the input F1 and the output F2, a clock extractor circuit CLKX, a delay line supplying a delay DEL, and an amplifier GM supplying the control power needed for the LiNO3 modulator MOD to operate. FIG. 1 also shows an input optical amplifier (EDFA) for alleviating the insertion losses of the regenerator circuit, a birefringent polarization control (PC) system and a band-pass filter (BP) for tightening the spectral distribution of the energy of the optical signals.
The prior art regeneration system has the disadvantage of requiring a clock recovery circuit, which is a costly component that cannot be integrated.
The problem arises even more acutely in the case of WDM signals, which require a regenerator, and consequently a clock signal recovery circuit, for each channel.
The present invention aims to alleviate the problems referred to above by proposing a WDM signal regenerator with no optical or electronic circuits for recovering the clock signal of the various WDM signals.
To this end, the invention provides a wavelength division multiplex optical signal regenerator adapted to be inserted between two optical fiber sections of a wavelength division multiplex fiber optic transmission system including a transmission line made up of a plurality of optical fiber sections and providing N channels at different wavelengths where N is an integer greater than 1, which regenerator includes:
a demultiplexer for demultiplexing wavelength division multiplexed optical signals onto N individual channels, and
a multiplexer for multiplexing optical signals at the output of the N individual channels into multiplexed optical signals,
wherein each individual channel includes:
a saturable absorber for reshaping the optical signals of each channel, and
a delay line,
and wherein the length of each delay line is chosen to introduce a total delay xcfx84nxe2x80x3 between the (n+1)th channel and the nth channel to obtain a time shift of xcfx84n between the (n+1)th channel and the nth channel at the output of the regenerator relative to the input of the optical fiber section of the transmission system, intended to be directly upstream of the regenerator, where xcfx84n is greater than zero.
We have surprisingly found that in a wavelength division multiplex fiber optic transmission system cross phase modulation (XPM) is the dominant phenomenon contributing to the jitter (uncertainty in the timing of the arrival of bits).
XPM is a multichannel effect in which phase modulation of one channel is induced by the intensity of the signal in one or more adjoining channels. This phenomenon therefore leads to distortion of the intensity of the signal to be transmitted through group velocity dispersion (GVD) and to uncertainty in the timing of the arrival of bits.
The regenerator as defined above introduces different delays into the individual optical signal channels to reduce XPM effectively, or even to eliminate it completely, which therefore considerably reduces jitter.
Thanks to the regenerators according to the invention, active retiming of the optical signals using regenerators which have a clock signal recovery circuit can be greatly reduced or even eliminated.
What is more, the regenerator according to the invention as defined above has the advantage that it can be integrated in a compact manner into a single component with a simpler configuration than regenerators with a clock signal recovery circuit.
The regenerator according to the invention can further include one or more of the following features:
xcfx84n has a value chosen to be less than a value eliminating the correlation of the intensity distortion contributions of each fiber section of the transmission system,
xcfx84n=xcfx84 for all the channels,
the time shift xcfx84 is less than approximately 600 ps,
the time shift xcfx84 is chosen to obtain destructive interference between the various distortion contributions of the individual optical fiber sections of the wavelength division multiplex fiber optic transmission system,
the shift xcfx84 is from approximately T/10 to approximately 1.5T where T is the duration of a bit,
the time shift xcfx84 is from approximately 5 ps to approximately 200 ps,
the multiplexer and the demultiplexer take the form of a combined multiplexing and demultiplexing unit with an array of waveguides in rows in which the saturable absorbers and the delay lines are disposed in the feedback loops, and
it includes a semiconductor optical amplifier in each channel in series with the saturable absorber.
The invention further proposes a wavelength division multiplex fiber optic transmission system comprising a transmission line formed of a plurality of optical fiber sections with a regenerator disposed between two optical fiber sections and wherein most regenerators of said system are regenerators according to the invention.